In this competitive renewal application, we will focus on the impact of HIV-1 replicative fitness as it relates to acute infection and subsequent disease progression. Our most recent data suggests that the mode of transmission (heterosexual/vaginal, intravenous blood, or MSM/anal-rectal) may select for different HIV-1 clones (in the blood). To pursue this hypothesis in a subtype B controlled background we have obtained acute HIV-1 infection samples from MSMs, hemophiliacs, IV drug users, and women infected via heterosexual contact. In aim 1, we will investigate the fitness of these acute isolate in dual or multi-virus competitions in T cells, macrophages and PBMCs as well as in explants of vaginal, cervical, rectal, and penile tissues (in collaboration with Dr. Shattock, Imperial College. We will utilize these new assays for receptor binding/usage and for replication kinetics to determine possible phenotypic differences in acute HIV-1 based on the mode of transmission (aim 1). We have also recently discovered that higher genetic diversity is maintained at the site of transmission (i.e. female genital tract or FGT) for several months and even when a single HIV-1 clone is found in the blood at acute/early infection. For aim 2, blood, and cervical swab samples are available from over 270 Ugandan (Ug) and Zimbabwean (Zim) women at acute/early infection. We will identified the transmitted blood strain as well as examine the genetic diversity in the blood and FGT using 454 pyrosequencing (aim 2). Fitness of transmitted HIV-1 clone(s) in the blood will then be compared to the fitness of those HIV-1 clones that continue to replicate and remain resident in FGT during early infection. In aim 3 we will then determine if the fitness of this transmitted blood clone (from aim 2) impacts on subsequent disease progression in the Ug/Zim women. Recent studies by our group and others have shown that most elite suppressors are infected with weak HIV-1 isolates. After ten years of study on the Ug/Zim natural HIV-1 history cohort, we have now confirmed our initial observations of slower disease progression (as related to CD4+ cell declines) in subtype C infected women over subtype A and then subtype D infected women. Remarkably, the slow disease progression and delay in viral load set point with subtype C infection reflects the poor ex vivo replicative fitnes of acute subtype C HIV-1 isolates in >20 patients. In aim 3, we will continue our analyses of viral load, CD4 cell counts, HIV-1 genetic diversity, and fitness. We now have sufficient data for advanced modeling studies to examine the impact of HIV-1 fitness (in combination with other host and viral parameters) on disease progression and virus spread in the human population. Based on strong reviewer encouragement from the last funding period, we will continue our modeling studies with our collaborators, Drs. Thomas Leitner and Alan Perelson at Los Alamos National Laboratories (aim 3).